Why is Google Maps API V.3 asking for an API key?

IT & Software | Other
Javascript API V.3 is connected to the project, according to the Google documentation, it is not necessary to prescribe an API key in the third version.

All this worked for several months and suddenly warning came up when entering the maps in the console: Google Maps API warning: NoApiKeys https://developers.google.com/maps/documentation/javascript/error-messages%20#no-api-keys 
Moreover, it is impossible to determine which part of the code causes this varning...there is no stack, and where the ears grow from is unclear.

How to understand what exactly this warning causes?
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As of June 22, 2016 Google Maps V3 no longer supports keyless access so you need to get a key for every (referrer-)domain which has never had a Google Map on it before.

and append it to the URL of the script like so:

<script src="https://maps.googleapis.com/maps/api/js?key=YOUR_API_KEY" type="text/javascript"></script>

on April 13th, 2020 (9:17 pm)
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